Auto-canning of radiation sources



y 4, 1966 T. N. LAHR 3,253,152

AUTO-CANNING OF RADIATION SOURCES Filed April 20, 1962 F IG.1

/ I 8 I g INVENTOR. THOMAS N. LAHR W4 SW ATTORNEY United States Patent 3,253,152 AUTO-CANNHNG 0F RADIATION SOURCES Thomas N. Lahr, Roseville, Minn assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Apr. 20, 1962, Ser. No. 189,098 6 Claims. (Cl. 250-106) This invention relates to new and surprising methods for encapsulating radioactive ceramic particles within a metal matrix and to the products so produced.

Heretofore, in order to make a doubly encapsulated radiation source using radioactive ceramic particles, it has been necessary to (1) fill an inner shell with radioactive ceramic particles, (2) seal said inner shell by means of brazing, welding or other method, (3) insert said inner shell into an outer shell, and then (4) seal said outer shell by a welding, brazing or the like. It has now been found that this prior art encapsulation process can be greatly simplified and the quality of the resulting encapsulated source greatly improved.

More particularly, this invention relates to a method for encapsulating radioactive ceramic particles which are thermally stable towards molten aluminum and molten zinc and which have average diameters between and 100 microns, comprising the steps of (1) forming a uniform physical mixture of said particles and a powdered metal having average particle diameters not larger than about 100 microns selected from the group consisting of aluminum and zinc, (2) placing the resulting mixture in a preformed shell of a metal thermally stable towards molten aluminum and molten zinc, (3) placing a plug of the same metal as the preformed shell in the mouth of the shell and then pressing the plug so as to compress the powdered mixture in the preformed shell to at least 90 percent of its theoretical density, and (4) heating the resulting sealed shell to a temperature sufiicient to fuse the metal powder.

The invention is better understood by reference to the attached drawing in which:

FIGURE 1 is a diagrammatic presentation of a shell filled with a uniform physical mixture of radioactive ceramic particles and aluminum or zinc powder.

FIGURE 2 diagrammatically shows the same mixture after it has been compressed and fired to a temperature sufficient to fuse the metal powder but below 1000 C.

FIGURE 3 is a diagrammatic cross-sectional view of the construction of FIGURE 2 taken along the line AA.

Radioactive ceramic particles suitable for encapsulation in accordance with the teachings of this invention are those which are stable at temperatures where aluminum and zinc fuse and become molten. Preferably the particles are thermally stable at temperatures below about 1000 C. in order to minimize any possible release of radioactivity to the metal matrix during the firing. The ceramic particle average diameters ranging from 10 to 100 microns. The aver-age diameters of the radioactive ceramic particles is said to fall within about 10 and 100 microns because particles of this size range are commonly used in this art, but those skilled in the art will appreciate that radioactive ceramic particles smaller or larger than these materials are also operable within the spirit and scope of this invention. The ceramic particles should have a chemical composition which is substantially inert towards molten aluminum and zinc. For example, suitable radioactive ceramic particles can be prepared by adsorbing a cationic radioactive isotope (e.g., cesium, strontium, etc.) on a particulate inorganic ion exchange of the type described in the Beck and Hann US. Patent No. 2,943,059 and subsequently fixing the radioactive isotope by a suitable heat treatment.

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Examples of suitable ceramic materials for use in this invention include zirconium pyrophosphate, alumina, zirconium silicate, zirconium oxide, or any refractory clay. Certain of these materials are more thermally stable than others, but in general all are suitable for use in this invention. A preferred ceramic material for purposes of this invention is zirconium pyrophosphate.

The radioactive ceramic particles are mixed with a powdered metal selected from the group consisting of aluminum and zinc. Such powdered metal has average diameters not larger than about 100 microns. As a matter of convenience, it is usually easier to employ metal powders which have smaller average diameters than those associated with the particular radioactive ceramic particles being used in forming a physical mixture of radioactive ceramic particles. Preferably the average metal particle diameter is less than 50 percent of the average particle diameter of the radioactive ceramic particles in any given mixture.

A physical mixture of metal powder and ceramic particles which is uniform (i.e., homogeneous) should be formed so that the distribution of radioactivity throughout the powder mixture is substantially constant. In this mixture the ratio of metal powder to ceramic particle is at least about 3:1 by volume, and preferably at least about 4: 1.

It is a surprising feature of this invention that only aluminum and zinc powder appear to be useful in the processes and products of this invention, for reasons not clear. Aluminum is much preferred.

The uniform physical mixture of radioactive ceramic particles and aluminum or zinc powder is introduced into a preformed metal shell. This shell is constructed of a metal which is inert towards molten aluminum and molten zinc and preferably is inert towards those metals at temperatures below about 1000 C. Preferred metal shell materials are monel or stainless steel. As will be seen from the discussion below, the shell should have sufficient strength characteristics to withstand the pressures developed during compression of the metal powder therewithin.

After the metal powder has been introduced into the mouth of the shell to a desired depth, the shell is plugged. The plug, like the shell metal, can be any conventional metal which is inert towards molten aluminum and molten zinc at temperatures below 1000 C. However, it is preferred for purposes of this invention to use a plug which has a composition similar to that of the shell in any given I case because the thermal coefficient of expansion of the plug should correspond to that associated with the metal shell. The diameter of the plug should correspond closely to the inside diameter of the shell mouth so that a tight fit may be had.

After the plug is in place, pressure is applied to its exterior or open face so as to compress the powdered mixture therebeneath. In general, sufficient pressure should be applied to the plug to compress the powdered mixture to at least percent of its theoretical density and preferably at least percent.

Next, the shell containing the compressed metal powder and its (preferably) fitted plug is subjected to heat. In general, the amount of heat used is that which is sufficient to fuse aluminum or zinc (depending upon whichever metal is used), but the temperature always kept below the limit of thermal stability of the radioactive ceramic particles used. Also, the temperature should not be maintained for an extended period of time at the fusion point so as tominimize any possible deleterious influence upon the radioactive ceramic particles. In general, times of less than 10 or even 6 minutes at fusion temperature are found to be sufiicient for purposes of this invention. One preferred temperature for the fusion of aluminum is 800 C., while a similar temperature for zinc is 600.

After heating, the mass is cooled. Optionally at this point the plug can be given as an after treatment a coating of sealant. Suitable sealants include solder, silver brazing alloy, or the like; the idea being that it is sometimes desirable to protect the interface between the plug and shell from deleterious atmospheric influences occasioned by the environment in which the particular encapsulated radioactive source of this invention will be used.

Reference to the figures at this point is helpful to understand the construction of the products of this invention. In FIG. 1, shell 10 containing uniform physical mixture 12 comprising radioactive ceramic particles 15 and metal powder is fitted with a plug 11 and compressed to at least 90 percent of its theoretical density. In these drawings only the radioactive ceramic particles are designated so as to show more clearly the migration effect which occurs during the heating operation.

In FIG. 2 the shell has been fired with the result that there has taken place a mysterious migration of the radioactive ceramic particles towards the central region of the shell. The result is that there is a ceramic particle free- Zone or region 9 within resulting inner metal matrix 14 which is substantially completely without any radioactive ceramic particles. This region always has a thickness of from 500 to 5000 microns (measured perpendicularly inwards from the inner wall of the shell). The reason why this particle-free region develops during the firing is unknown.

Because the powdered mixture before firing has not been compressed to theoretical density during firing, as those skilled in the art will appreciate, a void space 16 results when the metal powder particles fuse and coalesce. Usually this void space 16 is less than l and usually less than percent of the total volume of the space originally occupied by the unfired mixture of ceramic particles and metal powder. This void space in no way affects .the utility of this construction as a radiation source.

As was mentioned earlier, a sealant 17 is sometimes optionally employed upon the surface of the plug used in the shell. To aid in effecting this sealing action, it is desired to put a chamfer 18 on the plug 11. This chamfer 18 then becomes a receptacle for sealant material 17 in the final product.

The peculiar canning elfect observed in this invention by reason of the production of the particle-free zone 19 whereby there results centralization of the radioactive ceramic particles within the metal matrix is illustrated in the cross-section shown in FIG. 3. One can appreciate that the particle-free region 19 is relatively uniform being in no case less than about 500 microns, and in some cases as much as 5000 microns. Within this particle-free region 19 and securely fastened within the metal matrix 14 are the individual radioactive ceramic particles 15; hence, one has a doubly encapsulated source from the processes of the present invention comprising the shell and the particle-free zone 19. The radioactive ceramic particles themselves can be considered to be a third encapsulation, as those familiar with the art of making radioactive sources will appreciate.

The invention is further illustrated by reference to the following examples:

Example 1 A uniform physical mixture consisting of four parts by volume of aluminum powder having average particle diameters of about 10 microns and one part by volume of radioactive ceramic particles wherein the ceramic is zirconium pyrophosphate, having average particle diameters of about 50 microns, and containing 3300 millicuries per gram of Cs-137 is made up. Approximately 0.5 gram of this mixture is then filled into a monel right circular cylindrical shell with inside dimensions of three tenths inch in diameter by one inch in length. The average wall thickness of this cylindrical shell is 0.050 inch. A monel plug 0.20 inch diameter by 0.40 inch length is inserted into the shell opening and the mass is compacted to about 92 percent theoretical density by applying pressure with a small arbor press. This resulting construction is then placed in a muflie furnace and heated at 800 C. for 10 minutes. The construction is removed from the furnace and allowed to cool. Silver brazing alloy is then used as a sealant for the plug top in the metal shell.

The product is an encapsulated radioactive source containing the radioactive ceramic particles embedded in a solid aluminum matrix. Sections taken through the radioactive source show that 'the radioactive ceramic particles are uniformly dispersed throughout the aluminum matrix in all regions except that extending inwards from the inside of the shell wall for a distance ranging from about 800 to 1500 microns. The void space was equal to about 8 percent of the total original volume opposite the mixture of aluminum powder and ceramic particles before firing.

Example 2 A uniform physical mixture consisting of four parts by volume of zinc powder having average particle diameters of about 10 microns and 1 part by volume of radioactive ceramic particles wherein the ceramic is zirconium pyrophosphate having average particle diameters of about 50 microns, and containing about 3300 millicuries per gram of Cs-l37 is made up. Approximately 0.5 gram of this mixture is then filled into a stainless steel right circular cylindrical shell 0.30 inch inside diameter by 1.0 inch inside height. The average wall thickness of this cylindrical shell is 0.050 inch. A stainless steel plug 0.20 inch in diameter by 0.40 inch in height is inserted and the mass is compacted to about 95 percent theoretical density by the'use of a small arbor press. This cons ruction is then placed in a muffle furnace and heated for 10 minutes at a temperature of 600 C. The construction is then removed from the furnace and allowed to cool. Silver brazing alloy is used as a sealant for the plug top in the stainless steel shell.

The product is an encapsulated radioactive source containing the radioactive ceramic particles embedded in a solid zinc matrix. Sections taken through the radioactive source show that the radioactive ceramic particles are uniformly dispersed throughout the zinc matrix in all regions except that extending inwards from the inside of the shell wall for a distance ranging from about 600 to 900 microns. The void space was equal to about 8 percent of the total original volume opposite the mixture of zinc powder and ceramic particles before firing.

The claims are:

1. A method for encapsulating radioactive ceramic particles which are thermally stable toward molten zinc and molten aluminum and which have average diameters between 10 and microns, comprising the steps of (1) forming a uniform physical mixture of said ceramic particles and a powdered metal having average particle diameters not larger than about 100 microns, and selected from the group consisting of zinc and aluminum, (2) placing such resulting mixture in a preformed shell of a metal having a melting point above that of the melting point of the selected powdered metal, (3) closing the shell mouth with a plug of the same metal as the shell material, (4) pressing the plug so as to compress said powdered mixture in such shell to at least 90 percent of its theoretical density, (5) heating the resulting plugged shell to melt the entire mass of said metal powder, and (6) cooling the plugged shell.

2. A method according to claim 1, in which the powdered metal is aluminum.

3. A method according to claim 1, in which the powdered metal is zinc.

4. A radioactive source comprising metal-encapsulated radioactive ceramic particles consisting of a metal shell enclosing a metallic matrix characterized throughout its mass by coalesced condition characteristic of having been melted, chosen from the group consisting of aluminum and zinc, in which are embedded the said radioactive ce- 5 6 ramic particles, the matrix filling at least 90 percent of the 6. A radioactive source according to claim 4, in which volume inside said shell; and said radioactive ceramic th m tallic matrix is Zinc.

particles being uniformly dispersed throughout the said metallic matrix in all regions except that extending in- References Cited by the Examiner wardly from the inside of said shell Wall for a distance of 5 UNITED STATES PATENTS from about 500 to 5000 microns; and said ll a ing 3 2,870,341 1/1959 Pennock 250106 melting point above the melting point of that of the 2,975,113 3/1961 Gordon 250-106 X metallic matrix.

5. A radioactive source according to claim 4, in which RALPH NILSON Exammerthe metallic matrix is aluminum. 10 ARCHIE R. BO'RCHELT, Examiner. 

4. A RADIOACTIVE SOURCE COMPRISING METAL-ENCAPSULATED RADIOACTIVE CERAMIC PARTICLES CONSISTING OF A METAL SHELL ENCLOSING A METALLIC MATRIX CHARACTERIZED THROUGHOUT ITS MASS BY COALESCED CONDITION CHARACTERISTIC OF HAVING BEEN MELTED, CHOSEN FROM THE GROUP CONSISTING OF ALUMINUM AND ZINC, IN WHICH ARE EMBDEDDED THE SAID RADIOACTIVE CERAMIC PARTICLES, THE MATRIX FILLING AT LEAST 90 PERCENT OF THE VOLUME INSIDE SHELL; AND SAID RADIOACTIVE CERAMIC PARTICLES BEING UNIFORMLY DISPERSED THROUGHOUT THE SAID METALLIC MATRIX IN ALL REGIONS EXCEPT THAT EXTENING INWARDLY FROM THE INSIDE OF SAID SHELL WALL FOR A DISTANCE OF FROM ABOUT 500 TO 5000 MICRONS; AND SAID SHELL HAVING A MELTING POINT ABOVE THE MELTING POINT OF THAT OF THE METALLIC MATRIX. 